Field of the Invention
The present invention relates generally to global navigation satellite systems, and more particularly, to a method and devices for reducing an impact of shock and vibration on global navigation satellite system measurements.
Description of the Related Art
Global navigation satellite systems (GNSSs) can determine receiver location with high accuracy. Currently deployed global navigation satellite systems are the United States' Global Positioning System (GPS), the Russian GLONASS, the European GALILEO system, etc.
In a GNSS, a navigation receiver receives and processes radio signals transmitted by satellites located within a line-of-sight of the receiver. The satellite signals comprise carrier signals modulated by pseudo-random (PR) binary codes. These codes can additionally be modulated by binary information symbols.
The receiver measures the time delays of the received signals. For each satellite, the delay is measured relative to some scale. There is a rough scale to measure PR-code delay and, in addition, the delay in carrier phase can be measured. Such measurements have a small ambiguity interval (carrier period/cycle), but provide a higher accuracy. Carrier phase measurements are very important in differential navigation.
The following tracking systems are used: Delay Lock Loop (DLL) to track code delays and Phase Lock Loop (PLL) to track carrier phase delays. A few individual loops track each satellite.
An individual tracking channel is a structural unit of the receiver that measures a code delay and a carrier phase delay on a particular frequency from a satellite.
Processing the received satellite signal in each channel comprises accumulation of multiplication results of the signal (passed through the input and filtering circuits and frequency converter) by a reference carrier phase and/or reference code being generated in the receiver. The reference carrier corresponds to the carrier of the received signal for the given satellite, and the reference code corresponds to the PR-code of the same satellite. Circuits for multiplying and accumulating are called “correlators”, and the corresponding process is called correlation of two signals.
Many approaches to building receiver channels are known. The most specific is the structure of the individual channel with three paths and coherent generation of the reference carrier.
The first path calculates an in-phase correlation signal I. Such a signal is obtained if the first reference carrier is used, which is in-phase with the carrier of the input signal, and the reference code is a replica of the PR-code modulating the input signal.
If there is a phase correction of the reference carrier φ and a time shift of the reference code ε, the signal I isI=μUmR0(ε)cos φ,
Where μ is the binary symbol,
Um is the amplitude of the received signal, and
R0 (ε) is the cross-correlation function of the PR-code, passed through the receiver filter and the reference code, which is a locally-generated replica of the input PR-code.
The second path calculates a quadrature correlation signal Q. This signal is based on the second reference carrier which is π/2-shifted from the first reference carrier. Their correlation results in the quadrature component of the input signal. The reference code is the same as in the first path. Signal Q is presented as:Q=μUmR0(ε)sin φ
Correlation signals Q, I are used for building a PLL phase discriminator that outputs a mismatch signal Z for the closed loop of the tracking system. The most commonly used form of this dependence is Z=Arctg(Q/I).
A discriminator characteristic in the navigation receiver in the form of a mismatch signal-carrier phase dependence always includes a non-linearity. The dependence can be considered linear only at small mismatches in tracking mode. Bandwidths of the tracking loops should be chosen such that small dynamic errors at varying delays shall be provided.
The wider the bandpass, the smaller the dynamic errors. However, it is unacceptable to excessively expand the bandpass, since errors grow due to additive interference (including inherent receiver noise), and clock cycle slips and jumps become possible.
There are methods of resolving this problem by applying the total energy of all observed satellite to track common disturbances in a wider band (U.S. Pat. No. 6,313,789). The main idea there is generation of both individual PLLs tracking carrier phases of each of N satellites in view and of common loop tracking disturbances of carrier phases caused by the same causes for all the satellites.
Combining the common loop and N individual PLL circuits results in a multi-loop control system with new useful performance. Some external influences affecting the carrier phase are common for all channels. They are movements of receiving equipment and fluctuations of the reference generator frequency. These effects are mainly tracked by the common loop. Other external influences are individual and in practice independent in different channels. They are noise, most of atmosphere errors, frequency fluctuations of the onboard reference generator, and errors of anticipated satellite trajectory. They can be tracked by individual PLLs.
Since external common influences have a wider spectrum than that of individual ones, the common loop has to be made wider than individual PLLs. But common action of many satellites gives additional possibilities.
Implementation of the stable joint functioning of individual and common loops is contradictory and complicated enough. When high vibration and shocks with a wide spectrum are present, the common loop needs to be rapidly responsive and hence needs to have a high clock rate. The high clock rate can be unacceptable for the individual loop due to insufficient energy potential/power budget.
A common use of individual loops and common PLL loops has some danger of causing mutual influence of the individual loops. As a result, an inter-channel interference can occur, and loss of tracking in one PLL can cause tracking losses or jumps in PLLs of different channels.
Further improvement of the tracking system was proposed in commonly owned U.S. patent application Ser. No. 14/357,421, filed on May 9, 2014, incorporated herein by reference, describing a method of building a tracking system where a numerically-controlled oscillator (NCO) and a loop filter are common members of the individual and common tracking loops. It eliminated inter-channel interference and provided a stable tracking in a wide range of external dynamic disturbances.
U.S. Pat. No. 7,495,607 describes a method of building the navigation receiver with changeable status of satellite channels. Receiver adaptation to external conditions is achieved by switching guided and guiding PLLs tracking carrier phases of satellite channels, depending on the quality of the received input signal. When the number of simultaneously observed satellites is more than is needed, the adaptation provides independent operation of channels for those satellites whose signals at the current time are stronger and can be tracked by a wideband PLL. On the other hand, the adaptation allows tracking weak satellite signals thanks to narrow-band PLL, the operation of which is facilitated by compensating for interference due to strong signals.
Accordingly, a method and system for a more robust tracking of navigation signal's carrier phase is desired.